Paper and similar web-like material are coated by applying a coating mix onto the web surface which is then spread into an even layer using a doctor blade. In the coater, the web to be coated passes through a gap formed between the doctor blade and a suitable backing member, conventionally a rotating roll. The blade doctors or removes excess coating from the web surface and levels the coating mix into an even layer on the web surface. To achieve a layer with as even a thickness as possible, the gap formed between the web and the doctor blade should have as constant as possible a spacing in the cross direction of the web over its entire width. The linear force applied to press the doctor blade against the web should be high and constant over the entire width of the blade to attain an even spreading of the coating mix onto the web even at high web speeds.
For several reasons, the gap between the material web and the doctor blade cannot be maintained exactly constant along the width of the doctor blade. During machining, the doctor blade and its frame are fixed to the machining unit base with strong fixtures into a position that simulates their operating positions. Despite exact placement of the fixtures on the machining unit, defects will develop during fabrication of the doctor blade and its frame, thereby causing an error in the parallel alignment between the web surface and the doctor blade tip. Also, as the doctor blade of the coater is pressed against the moving web, a linear force is applied to the blade. Due to the pivotal support of the doctor blade frame provided by bearings mounted at both ends of the frame, the deflection induced by the linear force is greater at the center of the blade than at its supported ends. As a result, the spacing between the blade tip and the web is less at the edges of the web than at its center. Additionally, since the linear force exerted by the blade onto the surface of the web or the backing roll is less at the center than at the supported ends, any possible bumps on the web, as well as variations in the density and viscosity of the coating mix, can lift the blade tip away from the web.
To alleviate the aforementioned disadvantages, several different designs for the attachment of the doctor blade have been presented in the prior art. For example, a homogeneous loading of the blade over the entire web width has been attempted by means of a flexible blade combined with an adjustable blade holder assembly. In this example, the blade is attached to the blade holder so that the blade can be pressed against the web by means of a resilient element, for example, a pneumatically or hydraulically loaded rubber hose, which extends across the entire length of the blade. Because of the equal pressure prevailing in the hose along its entire length, the hose presses the blade against the web with a constant linear force over the entire width of the web. The blade pressure against the web can then be adjusted by altering the pressure in the hose. In this design, a doctor blade is occasionally used which is divided into narrow sections along its width. The advantage of this approach is a more flexible blade that offers an improved conformance with the shape of the web and the backing roll.
The above-described design has several disadvantages. Because of the limited deformation capability of the resilient loading element, this design is incapable of compensating for large variations in the spacing between the blade and the web, or for loading of the blade. The adjustment range of blade loading remains restricted, and, if a higher coating speed is desired, the blade must be pressed against the web with an actuator element attached to the doctor blade. A higher blade loading results in an increased stiffness of the blade holder element so that the blade becomes incapable of conforming to the web surface in a desired manner. Also, the frame of the doctor blade must be extremely stiff so that it can compress the flexible blade against the web.
Flexible and adjustable doctor blade holder constructions are complicated, blade changes are awkward, and damage to the pressure-exerting elements may result during blade changes. Consequently, the blade holder construction becomes large and heavy.
Calenders use deflection-compensated rolls having a load-bearing basic roll in the center of the roll. Pressure-exerting elements are placed between the basic roll and the shell of the roll so that when the shape of the elements is changed, the roll shell is straightened. A deflection compensated doctor blade beam based on a similar construction is described in U.S. Pat. No. 4,907,528 where the doctor blade beam has four pressure-exerting elements symmetrically positioned about a round frame beam and enclosed by a tubular shell which itself is supported to the square frame of the doctor blade assembly. By adjusting the operating pressure of the pressure-exerting elements, the frame of the doctor blade assembly can be deformed appropriately to compensate for the deflection of the doctor blade beam of the coater.
G.B. Patent No. 1,202,167 describes a similar doctor blade beam supported by a square coater frame containing an inner tube with a square box section. Between the inner tube and the coater frame are mounted pressure-exerting elements, which are attached on the two opposing sides of the coater frame. Thus, the beam deflection can be compensated in the direction of one bending axis by altering the pressure prevailing in the pressure-exerting elements.
However, the beam construction described in U.S. Pat. No. 4,907,528 is complicated, thereby resulting in a considerably high weight. As a result, the inherent weight of the beam additionally contributes to its deflection, thus requiring more powerful means of compensation. Furthermore, the shape of the beam is not freely selectable by the designer because the frame of the coater must necessarily have a square shape and because there must necessarily be four pressure-exerting elements. The tube connecting the pressure-exerting elements is joined to the coater frame by means of gliding shoe members, and, due to this supporting method, the coater frame and the gliding shoe members must be machined with great accuracy and have smooth gliding surfaces. Therefore, the construction becomes extremely expensive. Furthermore, the friction associated with a gliding support complicates the doctor blade beam compensation, and moreover, contributes to increased wear in such a construction.
The construction described in G.B. Patent No. 1,202,167 is relatively simple, but it is only capable of compensating the beam deflection in one bending axis.